Libyan opposition forces have rejected a ceasefire offer by Muammar Gaddafi and dismissed his regime's claims that loyalist forces had cut off access to the crucial seaport in the besieged city of Misrata.

In a rambling, defiant speech on state television on Saturday, in which he declared that he was "more sacred [to Libyans] than the emperor of Japan is to his people", Gaddafi called for talks with Nato, which is conducting air strikes against his forces.

"The door to peace is open," Gaddafi said. "You are the aggressors. We will negotiate with you. Come, France, Italy, UK, America, come, we will negotiate with you. Why are you attacking us?"

More than two months into the Libyan revolution, loyalist forces are becoming increasing stretched. In the east, they are preventing the rebel advance near the town of Ajdabiya; in the far west, they are trying to quell a more recent uprising near the border with Tunisia. And just 130 miles from Tripoli, the battle for the industrial city of Misrata continues, with at least six people killed before noon on Saturday.

In an echo of comments by his son Saif, who said on Friday the regime would fight on for 40 years if necessary, Gaddafi said his own future would not up for discussion in any talks.

"No one can force me to leave my country and no one can tell me not to fight for my country," he said.

Around the same time as he was speaking, Nato bombs stuck a government complex in Tripoli.

A Nato official told Associated Press that the alliance needed "to see not words but actions" from Gaddafi. Indeed, the regime's message seemed contradictory, with government spokesman Moussa Ibrahim saying on Saturday that Misrata would face "total fire" if its civilian army did not accept an amnesty offer within four days.

The opposition Transitional National Council, in the eastern city of Benghazi, which is under rebel control, said the ceasefire offer was not genuine and vowed to fight on.

"[Gaddafi's regime] has repeatedly offered ceasefires only to continue violating basic human rights, international humanitarian law, and the safety and security of Libya and the entire region," said Abdelhafed Ghoga, the council's vice-chairman.

"The time for compromise has passed. The people of Libya cannot possibly envisage or accept a future Libya in which Gaddafi's regime plays any role."

Having been forced out of the city centre in Misrata following weeks of heavy fighting against the rebels, loyalist forces equipped with tanks are now trying to defend their positions on the edge of the city. There were more heavy clashes near the airport, where some of the government forces are massed, as well as close to Zawiya Al-Mahjoub, a satellite town about eight miles west of Misrata.

Gaddafi's troops have also stepped up their attempts to take control of the port, which is the city's lifeline, allowing in food and other humanitarian supplies as well as light weapons from Benghazi.

Several days of shelling have affected the coming and going of ships, with boats carrying urgent supplies forced to stay a safe distance from the coast for long periods until the barrage stops. On Saturday morning, a ferry chartered by the International Organisation for Migration to pick up stranded migrants in Misrata was among the ships prevented from docking.

On Friday, Nato warships caught Gaddafi's forces trying to lay sea mines around the harbour. Libyan state television claimed – falsely – the port had been put "out of service" and said all humanitarian aid should be transferred "overland and under the supervision of the armed forces" – an impossible situation given that the city is under siege.

The rebels are keenly aware of the importance of the port and have put up strong defences around it. In clashes close to the harbour on Saturday morning, five rebel fighters were killed when their position was shelled.

Ahmed Abuhamik, 22, a rebel fighter who was stationed nearby, said Gaddafi's forces did not have the power to take over the port as long as Nato planes were striking their positions.

"Nato are trying, but we need them to hurry up. Too many civilians are dying," he said.

In his amnesty offer, Gaddafi claimed that rebels who gave up their guns would be offered cars and cash.

"Does he think this revolution is about money?" said Abuhamik. "It is about freedom. Anyway, if we give up our weapons, he will kill everyone here, trust me."